Step 1: Locate the content.
Part XI (Articles 245–263) deals with Relations between the Union and the States:
• {Chapter I} (Arts. 245–255) — Legislative Relations (territorial/subject-matter competence, residuary powers, repugnancy).
• {Chapter II} (Arts. 256–263) — Administrative Relations (obligations of States/Union, directions, Inter-State Council).
Step 2: Distinguish other Parts.
Part IX—Panchayats; Part X—Scheduled and Tribal Areas; Part XII—Finance, Property, Contracts and Suits (financial relations, but not the whole centre–state scheme).
Step 3: Conclude.
\[
\boxed{\text{Centre–State relations are set out in Part XI (Option C).}}
\]